Various types of hypodermic needles have been developed for the injection of substances, e.g., drugs, into the body, or the withdrawal of body fluid samples, e.g., blood. For drawing blood samples from a vein, the subject's blood pressure may be sufficient to pump the blood sample through the bore of the hypodermic needle to a container. In situations where a needle is used to withdraw samples of other body fluids, it is generally necessary to use some type of mechanism to draw or pump the body fluid through the needle to a container, for example, a plunger attached to the needle which can be manipulated to provide a sufficient vacuum draw on the needle to draw the fluid sample through the bore of the needle into the container of the plunger.
While conventional hypodermic needles that can be manipulated by hand by medical personnel have been successfully used for many years for withdrawing relatively large samples of blood and other body fluids, the use of such needles is, of course, accompanied by some tissue damage as the needle punctures the skin and subdermal tissues of the subject. This incidental tissue damage becomes particularly a problem if repeated sampling is required, for example, for monitoring of blood sugars by diabetics or in other situations where it is desirable to be able to closely monitor the levels of blood components or drug levels within a patient. Efforts have thus been directed to the development of miniaturized needle systems which are capable of very lightly puncturing the skin, and in some cases limiting penetration to the dermal layer of skin to collect interstitial fluid in the dermal layer without puncturing capillaries beneath the dermal layer. Examples of such systems are shown in, e.g., U.S. Pat. Nos. 5,582,184, 5,682,233, 5,746,217 and 5,820,570. The small size of the needles and the small amounts of interstitial fluids that are withdrawn through the needle complicate the problem of drawing fluids through the needle to a sample container or a sample chamber.
It has been found that ultrasonically actuated needles will pump fluid from the distal end of the needle to its proximal end under appropriate conditions. Such needles can also be fabricated using microelectromechnical fabrication techniques. See, A. Lal, et al., “Silicon Micro-Fabricated Horns for Power Ultrasonics,” Transducers 95, June 1995, Stockholm, Sweden; A. Lal, et al., “Micromachined Silicon Ultrasonic Needle,” IEEE Ultrasonics Symposium, Nov. 7-10, 1995, Seattle, Wash., U.S.A.; and U.S. Pat. Nos. 5,569,968 and 5,728,089 to Lal, et al. A limitation of the use of such ultrasonically actuated needles for body fluid sampling is the potential damping of the vibration of the needle as it is inserted into relatively stiff skin tissue. The damping of the vibrations of the needle by skin tissue reduces the efficiency of pumping of fluid from the needle. Further, increasing the amplitude of the vibrations applied to the needle to overcome the loss of efficiency may result in excessive disruption of the body tissue penetrated by the needle by virtue of the mechanical displacement, cavitation, and localized heating of tissue caused by the vibrations.